Denial of service (DoS) attacks on Internet sites are all too frequent occurrences, causing certain forms of malicious damage to affected computer systems. Generally, a DoS attack aims to prevent legitimate users from accessing computer services on such systems. For example, by overwhelming the network bandwidth of a business's web site, a malicious computer (or groups of computers) can prevent other users from accessing the business's web site and placing orders or obtaining services. Reasons for such attacks vary but there are recent attempts at blackmail, similar to a protection racket. In some circumstances, DoS attacks can also resemble “picketing” the business that owns the computer system.
Perpetrators can generate a DoS attack in a number of ways. Three basic areas of attack exist, although other attacks may be used:                (1) the consumption of limited resources, such as bandwidth, disk space or CPU time;        (2) alterations to configuration information, such as routing information or registry entries; and        (3) the physical disruption of networking components        
Attacks on resources have become increasingly popular, mainly through attempts to “flood” a network with excess or spurious packet data over the Internet, thereby preventing legitimate traffic to the web site. In a distributed denial-of-service (DDoS) attack, many computers work together to attack a target system.
Some DDoS attacks use distributed intermediary devices against a network endpoint (i.e., an end system). A potentially untrustworthy or hostile master controller has the ability to instruct a number of intermediary slave agents (e.g., routers or proxies) to send excessive network traffic (i.e., attack traffic) to an end system in the network (or to otherwise use some resource of that end system excessively). Where the attack traffic load generated from each slave is greater than the traffic load injected into the slaves by the rogue master controller or “perpetrator”, the attack is termed a “network amplification attack”.